[kotlin-spring][server] Feat: Allow implementation of arbitrary interface in DTOs (similar to x-implements from java-spring) (#21950)

* add basic implementation and tests

* improve test a bit

* modify kotlin-spring.md

* add x-kotlin-implements also to enum

* update samples & properly define implemented vendor extension

* use enum.getName() instead of hardcoded string as key in vendor extension map

* fix docs

* fix test openapi spec and test

* add samples for x-kotlin-implements

* add samples for x-kotlin-implements to proper output folder

* fix

* revert unwanted changes

* move to correct place

* fix mustache template

* add to samples-kotlin-server.yaml

* reuse 1 open api schema for everything. Add also case where interface extends interface.

* add warn logs when x-kotlin-implements-fields is used without x-kotlin-implements to improve usability

* remove unnecessary generated files

* remove unnecessary generated files

* remove "status" inner enum from Pet as it fails to properly import as Pet.Status in implementations. This is a separate bug - not caused by x-kotlin-implements

samples/server/petstore/kotlin-springboot-integer-enum/src/main/kotlin/org/openapitools/model/ApiError.kt

@@ -27,7 +27,7 @@ data class ApiError(
 
     @field:Valid
     @get:JsonProperty("reasonCode") val reasonCode: ReasonCode? = null
-    ) {
+) {
 
     /**
     *